1. Field of the Invention
This invention relates to a method for forming a crystal, particularly to a method for forming a crystal by utilizing the difference in nucleation density (.DELTA.ND).
The present invention may be suitably used for, for example, formation of crystals such as single crystals, polycrystals, etc. which can be preferably used for functional devices such as electronic devices, optical devices, magnetic devices, piezoelectric devices and surface acoustic devices of semiconductor integrated circuits, optical integrated circuits, magnetic circuits, etc.
2. Related Background Art
In the prior art, single crystal thin films to be used for functional devices such as electronic devices optical devices, etc. using semiconductor materials have been formed by epitaxial growth on a single crystal substrate. For example, on a Si single crystal substrate (silicon wafer), St, Ge, GaAs, etc. have been known to be epitaxially grown from liquid phase, gas phase or solid phase. Also on a GaAs single crystal substrate, a single crystal such as of GaAs, GaAlAs, etc. has been known to be epitaxially grown. By use of a crystalline semiconductor thin film thus formed, a semiconductor device, an integraton circuit, or an emission device such as semiconductor laser, LED, etc. is prepared.
Also, recently, research and development of an ultra-high speed transitor by two-dimensional electronic gas, ultra-lattice device utilizing quantum well, etc. has been active done, and these techniques have been made possible by the high precision epitaxial techniques such as MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition) by use of ultra-high vacuum.
In such epitaxial growth on a single crystal substrate, it is necessary to match the lattice constants and the thermal expansion coefficients between the single crystal material and the epitaxial growth layer. If this matching is insufficient, lattice defects will developed in the epitaxial layer. Also, the elements constituting the substrate may be sometimes diffuse into the epitaxial layer.
Thus, it can be understood that the method of forming a single crystal semiconductor thin film of the prior art by epitaxial growth depends greatly on its substrate material. Mathews et al examined combinations of the substrate materials and epitaxial growth layers (EPITAXIAL GROWTH, Academic Press, New York, 1975, ed. by J. W. Mathews).
Also, the size of the substrate is presently about 6 inches, in the case of Si wafers. In view of productivity and low cost, using enlarger of GaAs and sapphire substrates is further not favored. Besides, a single crystal substrate is expensive and therefore the cost per chip becomes high.
Thus, for forming a good quality single crystal layer of capable of forming a functional device having excellent characteristics according to the prior art, there has been the problem that the substrate materials are limited to a very narrow scope.
On the other hand, there have been actively resarches and development of three-dimensional integrated circuits which accomplish higher integration and higher functions by forming semiconductor devices in the normal direction of the substrate by lamination. Also researches and development on large area semiconductor devices such as solar batteries having devices arranged in an array on an inexpensive glass, liquid display devices provided with switching transistors, etc. are becoming abundant year by year. I
What is common to both of these they require a technique to form a semiconductor thin film on an amorphous insulating material and form an electronic device such as transistor, etc. there. Among them, particularly it has been desired to have a technique for forming a single crystal semiconductor of high quality on an amorphous insulating material.
Generally speaking, when a thin film is deposited on an amorphous insulating material substrate such as SiO.sub.2, etc., due to deficiency of long distance order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline. Here, amorphous film is one under the state where, although the short distance order to the extent of the minimum approximate atoms is preserved, there is no longer distance order than that. While polycrystalline film is a collection of single crystal grains having no specific crystal direction separated by grain boundaries.
For example, when Si film is formed on SiO.sub.2 by the CVD method, if the deposition temperature is about 600.degree. C. or lower, it becomes amorphous silicon film, or if it is higher than that temperature, it becomes a polycrystal silicon with a distribution of grain sizes from some 100 to some 1000 .ANG.. However, the grain sizes and the distribution thereof will vary greatly depending on the formation method.
Further, a polycrystlline thin film of large grain sizes of about micron or millimeter has been obtained by melting and solidifying an amorphous film or polycrystalline film with an energy beam such as laser, rod-shaped heater, etc. (Single-Crystal silicon on non-single-crystal insulators, Journal of crystal Growth vol. 63, No. 3, October, 1983, edited by G. W. Cullen).
When a transistor is formed on the thin film of each crystal structure thus formed and the electron mobility is measured from the actuation characteristics of said transistor, a mobility of ca. 0.1 cm.sup.2 /V.sec is obtained in the case of amorphous silicon, 1 to 10 cm.sup.2 /V.sec in the case of a polycrystalline silicon film having an average grain size of some 100 .ANG., and a value to the same extent as in the case of single crystal silicon film in the polycrystalline silicon film with enlarged grain sizes by melting and solidification.
From these results, it can be understood that the device formed in the single crystal region within the crystal grain and the device formed as bridging over the grain boundary differ greatly in their electrical characteristics. More specifically, the deposited film on an amorphous material obtained by the prior art method becomes amorphous or polycrystalline structure having a grain size distribution. The semiconductor device formed in such a deposited film becomes Greatly inferior in its performance as compared with the semiconductor device formed in a single crystal film. For this reason, the uses are limited to simple switching devices for which no high conversion efficiency is demanded, such as solar batteries or photoelectric converting devices, etc.
Also, the method of forming a polycrystalline thin film with large grain sizes by melting and solidification had the problem that it took a very long time enlarge grain sizes and was poor in productivity, because amorphous film or polycrystalline thin film is scanned with energy beam for each wafer. It was also not suitable for enlargement of area.
Also, within the polycrystalline film thus formed, there exist randomly a large number of grain boundaries, which cause poor characteristics when a semiconductor device is prepared by use of said polycrystalline film. Therefore, it has been strongly desired to have a method which can form a single crystal with high single crystallinity or a polycrystal which easily controls the position of the grain boundary and at low cost even on an amorphous film.
Accordingly, the present Applicant has proposed European Published Patent Application 244081 which discloses a method of providing a single crystal film containing no grain boundary, a polycrystal controlled in grain size or the position of grain boundary, etc., without restriction of the base material for forming the crystal, for example, without restriction of the material, the chemical composition, size, etc. of the substraate.
The above European Published Patent Application No. 244081 effects selective nucleus formation by utilizing the difference in nucleation density (AND) in the two kinds of materials with different nucleation densities (ND) depending on the place on the deposition surface relative to a certain deposition material (e.g. Si). Therefore, as the difference in nucleation density (.DELTA.ND) between two kinds of materials is greater, better selectivity can be obtained for selective nucleus formation.
The technical meanings of the words "nucleus", "nucleation surface", "nonnucleation surface" as mentioned in the published specification and the present application are as follows.
"Nucleus" refers to a stable nucleus having a size greater than the critical nucleus (the nucleus of which free energy becomes the maximum). Unless indicated otherwise, "nucleus" refers to a stable nucleus in this specification.
"Nucleation surface" refers to an artificial surface of fine area possessed by the substrate on which crystal growth treatment is applied for forming a stable nucleus and permitting a single crystal to grow from a single nucleus. It is formed of a material with large nucleation density.
"Nonnucleation surface" refers to a surface of the region for permitting no crystal to grow even if a nucleus is generated possessed by the substrate on which crystal growth treatment is applied, which covers substantially the whole surface of at least one surface of said substrate. It is formed of a material with small nucleation density.
For example, to describe by referring to FIG. 1, when SiO.sub.2 is used for forming the nonnucleation surface material and Si.sub.3 N.sub.4 for forming the nucleation surface material, it can be seen from the graph that the difference in nucleation density (.DELTA.ND) between the both is about 10.sup.2 -fold relative to St. Substantially good selective nucleation is possible by use of the materials having such extent of difference in nucleation density (.DELTA.ND), but there may sometimes occur generation of a nucleus not only on the nucleation surface arranged at the desired position on the substrate but also on the nonnucleation surface (Error Nucleation). If the whole nucleation density (ND) is lowered too much by, for example, increasing the amount of HCl added for inhibiting such Error nucleus, there may occur such phenomenon as no formation of nuclues on the Si.sub.3 N.sub.4 nucleation surface which is the, whereby yield is lowered.
Accordingly, to describe the case of utilizing SiO.sub.2 for the non-nucleation surface and SiO.sub.2 doped with Si ions for the nucleation surface, respectively, as shown in FIG. 1, one having Si ions implanted at a dosing amount of 2.times.10.sup.16 cm.sup.-2 implanted on SiO.sub.2 has a difference in nucleation density (.DELTA.ND) of about 10.sup.3 -fold from SiO.sub.2 itself, and an extremely large difference in nucleation density (.DELTA.ND) can be created at a greater dosing amount. Therefore, even if the whole nucleation density (ND) may be lowered by controlling the amount of HCl gas added, nuclei are generated with good yield at the nucleation surface to form a single crystal grown from a single nucleus, and also generation of Error nucleus can be sufficiently inhibited at the non-nucleation surface, whereby formation of a crystal on the nonnucleation surface can be prevented. Thus, it can be understood that the above "ion implantation method" (hereinafter written as I/I method) is a very effective means for the selective nucleation method.
However, when the I/I method is practically practiced, the following inconveniences were found to occur in some cases. This is described by referring to FIGS. 2A-2C. First, for implanting ions in a fine nucleation region, the whole non-nucleation surface is required to be covered with a resist having a window opened at the fine nucleation region as mentioned above, and Si ions are implanted in the whole region including the nucleation surface and the nonnucleation surface (resist surface) (FIG. 2A). Next, the resist is removed with the use of an organic solvent, etc. (FIG. 2B). Whereas, at this time, only by implanting Si ions, something which appears to be denatured resist product remains in some cases. The above denatured product, according to the experiment by the present inventor, was sometimes generated when Si ions were implanted at a dosing amount of 3.times.10.sup.16 cm.sup.-2 or higher. Also, the denatured product contained carbon as the result of surface analysis, and it may also be estimated to remain in the form such as SiO. The above denatured product is a substance having very high nucleation density, and when selective nucleation is carried out, it has been also found that an Error nucleus may be sometimes formed with the denatured product as the center and grown to a crystal (FIG. 2C).